In a color picture tube having a shadow mask, the mask is arranged in direct proximity to the interior surface of the screen of a picture tube. Because luminescent segments are produced on the interior surface of the screen, the geometry of the shadow mask typically conforms to the pattern of the luminescent segments when the color picture tube is in operation. Maximum impact accuracy of the electron beams on the luminescent segments is achieved when the aperture geometry of the shadow mask matches the distribution of the luminescent segments on the interior surface of the screen at a predetermined operating temperature. However, only a small portion of the emitted electrons pass through the mask and strike the luminescent segments. The majority of the electrons strike the mask directly. Consequently, the mask can heat up to 80.degree. C., thereby resulting in a change in mask geometry.
This change in mask geometry can produce a doming effect in the mask. The aperture geometry of the shadow mask no longer conforms to the distribution of the luminescent segments, resulting in imprecise electron strikes. The color rendering quality of the screen can be affected.
With high contrast pictures, different areas of the mask can be heated to different levels, thus giving rise to partial doming of the mask (local doming) which can result in aberrations when a predetermined tolerance level is exceeded.
Various attempts have been made to limit or prevent this disadvantageous thermal behavior, which can affect the shadow mask. In addition, various measures have been suggested to limit excessive heating of the mask.
U.S. Pat. No. 3,887,828 suggests arranging a thin layer of metallic aluminum and a manganese dioxide layer onto a metallic apertured mask. The aluminum layer is in contact with the apertured mask at the aperture edges only. It should have electrically conducting and electron-absorbing properties. Another layer of graphite, nickel oxide or nickel iron is coated on top of the aluminum layer.
In the '828 patent, the porosity of the manganese oxide layer is said to originate substantially from the individually arranged particles, which layer forms a sandwich-like structure with the thin aluminum layer. Due to this layered structure, heat generated by the impact of electrons is intended to be kept away from the metallic apertured mask and emitted in the opposite direction.
This solution has various drawbacks. It was found that keeping the generated heat away from the apertured mask is not feasible, because the majority of heat is not generated within the aluminum layer and the overlying graphite layer, but in the apertured mask. The electron-reflecting, electron-absorbing and heat-emitting properties of the aluminum layer are too low. The heat insulating sandwich structure arranged on top of the apertured mask can emit heat, but only with difficulty.
Furthermore, there may have been suggestions to provide the surface of an apertured mask with a heat-insulating layer and to coat a cover layer containing heavy-metals on top thereof. The heat-insulating layer consists of porous solids which are coated on the metallic apertured mask together with a binder. However, the technological input of coating two layers, namely, one heat-insulating layer and one cover layer containing heavy-metals arranged on top thereof, is relatively high.